Transcript How Cells Divide

Cell Division
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Why do cells divide?
The Importance of Cell Division
1.
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Healing and Tissue Repair
Replacement of dead cells with new ones
E.g. peeling dead skin after a sunburn
E.g. injuries (cut, pulled muscle, broken
bone)
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Few cells live for the adult lifespan of a
multicellular organism
Skin cells are replaced quickly while brain
cells have a very limited reproductive capacity
Aging - cells are dying more quickly than
they are replaced
aside
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Scars form as the result of injury to the dermis.
The dermis is a layer of skin that is both deep
and sensitive. When damage to this layer of
skin occurs, the body produces new collagen
fibers for the purpose of mending the injury.
When the wound is finished healing, a scar is
formed.
The Importance of Cell Division
cont’d
2.
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Growth
Cell division increases the number of cells
Allows for an increase in the size of the
organism from baby to adult
So why can’t cells just
grow bigger?
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As a cell grows, the volume of cytoplasm
increases faster than the surface area of the cell
If a cell becomes too large, there would not be
enough exchange of materials (oxygen, food)
through the cell membrane to sustain it
Also, the nucleus would be too far from all the
other organelles
Makes sense right?
The Importance of Cell Division
cont’d
Reproduction of Some Organisms
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Asexual Reproduction
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Form of genetic reproduction that only
requires one parent
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Offspring have the exact DNA as the parent
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E.g. Yeast, Bacteria,
and Protists (amoeba)
3.
How Cells Divide
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Cells go through a process called Mitosis
Mitosis is part of the cell cycle
There are three distinct stages of cell growth
and division
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Interphase
Mitosis
Cytokinesis
The Cell Cycle
1.
2.
3.
Interphase
Consists of 3 stages:
A) Growth 1 (G1)
B) Synthesis (S)
C) Growth 2 (G2)
Mitosis- 4 stages (PMAT)
Cytokinesis- the actual splitting of the cytoplasm
The Cell Cycle
Stage 1 - Interphase
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Longest duration of cell stages
Cell carries out life activities (growth, cellular
respiration)
Genetic material (DNA) is in very long, thin
strands called chromatin. They are invisible.
Just before mitosis, the DNA will replicate
forming two identical strands.
Interphase
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Cells remain in interphase if:
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There are not enough nutrients to provide cell growth
DNA has not been replicated
DNA is damaged
Signals form the surrounding cells tell it not to divide
DNA may be repaired if it is early in the cell
cycle
When there is too much damage – cell will be
destroyed
Stage 2 -Mitosis
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There are four phases in mitosis
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Prophase
Metaphase
Anaphase
Telophase
1st: a few more cell parts
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There are some important features/organelles
involved in mitosis:
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Centrioles-made of microtubules which direct
spindle fiber production
Spindle fibers- link centrioles to chromatids
Chromatids – duplicated chromosomes
Centromeres- link sister chromatids
Telomeres- ends of chromosomes
Prophase
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The long strands of
DNA condense into
compact forms that
are visible –
chromosomes
Chromosomes have
two identical halves
called sister
chromatids
Nuclear membrane
dissolves
Metaphase
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Chromosomes line up in the middle of the cell
Spindle fiber joins the centromere to the centriole at
the edge of the cell
Anaphase
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Sister chromatids are pulled apart and migrate to
opposite sides of the cell.
Become daughter chromosomes
Telophase
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Chromosomes reach ends of cell
Nuclear membrane reforms around
chromosomes
Chromosomes begin to uncoil (decondense)
Cleavage furrow forms
Stage 3 - Cytokinesis
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Cytoplasm and organelles are divided between
the two nuclei
Cell membrane pinches off producing two
identical cells (daughter cells)
Different in plant cells- instead, a cell plate is
formed
Plant Cell Division
Animations
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http://0.tqn.com/d/biology/1/0/d/_/mitosis.gif
http://www.cellsalive.com/mitosis.htm
http://www.sumanasinc.com/webcontent/anim
ations/content/mitosis.html